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89 BII 2.9L Oil Pressure Drop

DoubleB89

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Automatic
After driving my BII for roughly 20 mins or so the oil pressure drops to where the engine will cut out at stop lights (almost like it is stalling). This problem is avoided by putting the deuce into neutral (automatic). I just changed the oil in this thing, and had this issue prior to my last oil change. I just put Castrol full syn 5-w30 into the motor this last weekend. (it had 10-w40 and that didnt have any pressure issues, but developed the 2.9 click at that point)

I am not the most motor inclined but perform all mait on this truck that i have had for about 9 months now (tires, battery hookups/battery, cooling system, oil changes). I want to have this thing up to par since I will be relocating from NC to FLA next year. When I get their I will be looking to replace the motor with a 4.0, but want to keep this thing running before then. Is there any advice you guys have?
 


RonD

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Clean the IAC(idle air control) Valve on the upper intake, computer uses this to set idle.
Automatic should have 750-800rpm when stopped and in gear

Your problem isn't oil pressure, that is just a symptom.

You RPMs are dropping below 750 when in gear and stopped, i.e. (almost like it is stalling)

'89 Ford will have an oil pressure switch, so on or off, when oil pressure is above 6psi switch is ON and you see oil pressure on the gauge, if below 6psi switch is OFF and you see no oil pressure on the gauge.

In general you will get 10psi of oil pressure per 1,000rpm, between 0-4,000rpm
So 600rpm = 6psi
Synthetic can act a bit thinner at low RPM so switch could be going OFF under 700rpm
Oil pressure is "back pressure" from the oil passages and bearings, the oil pump pushes too much oil into the engine, the oil can't get through the passages or out the bearings fast enough so "back pressure" is built up in the main passage where oil pressure switch is located.
5-30 is thinner oil so can get pushed through the passages and bearings faster, so would always show up as lower oil pressure, than 10-40, and on older engines with more wear on the bearings(larger gaps) oil can flow out easier as well.
But as long as lifters are not tapping you have enough oil pressure for engine to be cooled and lubed.

To test IAC valve.
Cold start, don't touch gas pedal
idle should go up to 1,500rpm(IAC valve opened all the way), then drop down to about 1,100rpm(computer closes IAC valve a bit)
As engine temp warms up idle will come down, computer is closing IAC valve a bit more
(You do not need to touch the gas pedal for this to happen, there is no "choke cam" on the throttle like on a carb)

Once engine has warmed up idle should be about 700-750rpm(manual trans would be 600-650).
Then you put automatic in gear the computer gets the "in gear" signal from the NSS(neutral safety switch) on the trans, it will bump up the idle a bit after you shift into gear, same as it would if you have AC and turn it on.

After engine is warmed up and idling(in Park), unplug the IAC Valves wires, IAC Valve will close and idle should drop to 500rpm or engine may even stall, either is good it means IAC valve is working, if idle stays high then either someone has messed with the TPS(throttle position sensors) idle voltage screw, or you have a vacuum leak.

TPS(throttle position sensors) idle voltage screw "looks" like an idle screw on the throttle linkage, while it "looks" like a carbs idle screw that is NOT why it is there, it is there to fine tune TPS idle voltage and as an anti-dieseling mechanism.

A small vacuum leak increases idle but it also Leans out the idle fuel mix, this will cause stalling out or stumbling RPM while stopped and in gear.


As a long shot, your '89 auto will have a Torque converter lock up solenoid(TCC), computer locks the torque converter once you are moving and then unlocks it when stopping, if TCC is not unlocking engine would stall when stopping, but not exactly what you described.
 
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DoubleB89

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Thanks RonD for the timely response.

The idle does exactly what you said in your post. I

I will be taking that apart and looking at it this weekend. Is there a specific cleaner that you would recommend for the Job? Or any old carbon cleaner?
 

RonD

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Good pictures in this thread: http://www.explorerforum.com/forums/showthread.php?t=84220

The IAC valve is a Controlled Vacuum leak, it's 2 ports by-pass the throttle plate allowing computer to adjust Filtered air flowing into the intake.
All fuel injected engines(not just Fords) need to use an IAC Valve, in some form, because there are no Jets, like in a carb.
In a carb system when you adjusted the idle screw to allow more, or less, air in it also pulled more, or less, fuel from the jets so the 14:1 air:fuel ratio stayed balanced.
Same with a Choke plate function on a carb, jets are needed to maintain correct air:fuel balance, richer mix in the case of cold engine using choke plate, choke plate increases vacuum in carb causing more fuel to be pulled from the jets, so richer mix.

With fuel injection none of that is possible, so to maintain the correct air:fuel ratio the fuel injection computer needs to control idle and to known the engine temperature to run engine richer with higher idle when cold(ECT sensor).

IAC Valve uses a Step Motor, (Google: Stepper Motor), these have 100 or so "steps" or stops, the steps are degrees of rotation the motor will turn to.
Step 10 might be a 1/4 turn of the motor
Step 70 might be 2 full turns
Each step can be "called" on by the computer sending the Step Motor a voltage Pulse, a voltage "Morse Code" if you will, so it is a "Digital" motor in that respect.
The computer "learns" what step to "call" to set engine at 700rpm, as an example, and what step to call for 1,100rpm(cold engine).
If you unhook the battery long enough or reset the computer, you will often hear the idle RPMs changing, as the computer "relearns" the Steps to use to get to it's pre-programmed RPM targets.
Installing a new IAC Valve would do the same.
 

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